U.S. Pat. No. 5,802,236 issued on Sep. 1, 1998 to D. J. DiGiovanni et al. (hereinafter DiGiovanni) describes non-periodic, microstructured optical fibers that guide radiation by index guiding. This patent is incorporated herein by reference. By appropriate choice of core region and cladding region, DiGiovanni discloses that the effective refractive index difference .DELTA. between core region and cladding can be made large, typically greater than 5% or even 10% or 20%. Such high .DELTA. allows for small mode field diameter of the fundamental guided mode (typically &lt;2.5 .mu.m), and consequently high radiation intensity in the core region. Illustratively, the fiber has a solid silica core region that is surrounded by an inner cladding region and an outer cladding region. In one embodiment, the cladding regions have capillary voids extending in the axial fiber direction, with the voids in the outer cladding region having a larger diameter than those in the inner cladding region, such that the effective refractive index of the outer cladding region is greater than that of the inner cladding region. DiGiovanni also discloses that non-periodic, microstructured fiber of this type has potentially many uses; e.g., as dispersion compensating fiber (with or without dispersion slope compensation), as amplifying fiber, as a laser, as a saturable absorber, for fiber gratings, and for non-linear elements.
From the standpoint of dispersion compensation, FIG. 6 and the associated description at col. 5, lines 61 et seq. of DiGiovanni disclose a computed group velocity dispersion spectrum of an exemplary microstructured fiber at infrared wavelengths of about 1515-1600 nm. As shown in FIG. 5, the fiber includes a silica core and air capillary cladding features. The negative dispersion spectrum of this fiber (solid curve 61) is compared to the positive dispersion spectrum (dashed curve 62) of a commercially available 5D.RTM. transmission fiber. About 1 km of the DiGiovanni fiber essentially perfectly compensates the positive dispersion of 94 km of a conventional single mode transmission fiber over a spectral range of more than 20 nm, about 50 nm.
DiGiovanni, however, does not describe the fiber dispersion of microstructured fibers at shorter wavelengths and, in particular, provides no description of such fibers for operation at the visible and near-infrared wavelengths. On the other hand, standard, single-mode fibers exhibit large normal (i.e., negative) group velocity dispersion in the visible wavelength region, severely limiting nonlinear optical interactions in this part of the electromagnetic spectrum. Yet there is a need in the art for devices and systems that exhibit relatively large nonlinear interactions at visible and near-infrared wavelengths. Hereinafter, the term vis-nir wavelengths will be deemed to include the visible spectrum from violet to red (i.e., from about 300 to 900 nm) as well as the near-infrared spectrum (i.e., from about 900 nm to less than about 1270 nm for silica). The upper bound of the vis-nir range may be different for other materials.